Redox behaviour of copper(II) tetrathia macrocyclic complexes

Abstract

Electrochemical redox properties of copper(II) complexes with quadridentate macrocyclic ligands [12]aneS 4, [13]aneS 4 and iso[14]aneS 4 have been investigated. The cyclic voltammetry at a platinum electrode in acetonitrile shows in all cases a quasi- reversible CuL 2+/+ electrode reaction with a half-wave potential of 0.96–1.15 V versus SHE. The reversibility of the redox process was confirmed also coulometrically. The size of the cavity in the 14- membered ring as well as its conformation flexibility permits a tetrahedral environment and leads to the stabilization of the oxidation state of Cu(I) more than in the 12- and 13-membered ring systems. All three copper(II) complexes undergo autoreduction in acetonitrile solution. The reaction is of the first order with respect to the copper complex with the rate constant (1.9–3.2)×10 −5 s −1. Due to the change of the polyhedron structure the activation energy of Cu(iso[14]aneS 4) 2+ autoreduction is higher than that of complexes with smaller rings. The formal potentials as well as the process of autoreduction of copper(II) tetrathia macrocyclic complexes model the reactivity of ‘blue’ copper proteins.